Method for the primary control in a combined gas/steam turbine installation

ABSTRACT

The aim of the invention is to improve a method for the primary control so that reserve power that is practically completely available for the effective frequency boost within seconds is available also in the stem turbine part of a gas/steam turbine installation. To this end, the pressure stage is operated with a control valve ( 6, 7, 8 ) that is throttled to such an extent that a frequency boost power reserve is built up. Said power reserve is used for frequency boosting in the event of an underfrequency by correcting the desired value depending on the underfrequency. Said corrected value corresponds to an effective area of the flow that is increased vis-â-vis the throttled condition of the control valve ( 6, 7, 8 ) and acts on the effective area of flow of the control value ( 6, 7, 8 ) with an impressed signal that approaches zero after a predetermined time. Said signal is chosen in such a manner that, despite the correction of the desired value, a stable operative condition is maintained in accordance with the response behavior of the gas/steam turbine installation to the increased effective area of flow.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/E-P01/07653 which has an Internationalfiling date of Jul. 4, 2001, which designated the United States ofAmerica and which claims priority on German Patent Application number EP00115684.3 filed Jul. 21, 2000, the entire contents of which are herebyincorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a method for primary control in acombined gas and steam turbine installation.

BACKGROUND OF THE INVENTION

A gas and steam turbine installation has a gas turbine part and a steamturbine part with at least one pressure stage. The working steam of thesteam turbine part is generated by one or more waste-heat boilers fedwith the waste heat from the gas turbine part. The pressure stage has atleast one steam turbine control valve. In this arrangement, the controlvalve passage cross section is adjusted by means of a control systemwhose required value is formed by the use of a control parameter,relating to power, of the pressure stage. A control parameter relatingto power means that the control system permits control of the power ofthe pressure stage. In this arrangement, the control parameter relatingto power can also be the power of the pressure stage itself. This willbe considered in more detail later.

A discrepancy between the instantaneous main system frequency and arequired main system frequency is determined in a frequency controlsystem and counteracting control action is taken as compensation for thediscrepancy.

Main system operators must guarantee fundamental mains systemoperational properties. This, in particular, also includes a certaintemporal electrical frequency (Europe: 50 Hz) which is stable withrespect to the electrical power demanded. Discrepancies from this areonly tolerated within certain narrow limits. The frequency stability inthe main system is ensured by use of dynamic load/power compensation.For this purpose, substantial reserve power must be available withinseconds. The main system operators must be able to offer this reservepower as a service.

In the case of combined gas and steam turbine installations, thisreserve power has previously been made available by the gas turbine partof the installation. Combined gas and steam turbine installations areinstallations in which waste-heat boilers are connected downstream ofthe gas turbines in order to operate a steam turbine installation. Inthis arrangement, the exhaust gas temperature of the gas turbines isgenerally kept constant over a wide power range. In such operation,however, there are limits to the change in the gas turbine power. Theload-changing capability is essentially limited by the dynamics of theexhaust gas temperature control and, therefore, by the ability to changethe air mass flow through the gas turbine. The steam turbine part of theinstallation generally follows the power changes of the gas turbineswith the substantially more sluggish time response of the waste-heatprocess. In the case of combined gas and steam turbine installationsusing frequency control, the change in the power demanded by the mainsystem is currently, therefore, brought about exclusively by the gasturbine part of the installations, because the steam turbine part makesno contribution in the initial seconds. The water/steam circuit, i.e.the steam generation and the steam turbine, is only a passive part ofthe overall gas and steam turbine installation and acts only as awaste-heat recovery unit.

In consequence, the gas turbine alone must provide the total reservepower in the case of primary control. During the operation of theinstallation, therefore, the frequency control reserves have to beincluded in the calculations for the gas turbine alone and the blockpower in steady-state operation is then reduced by a correspondinglyhigh proportion.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to provide amethod which, in the case of the gas and steam turbine installation,also permits the provision of reserve power in the steam turbine part,which reserve power is almost completely available within seconds foreffective frequency maintenance.

An embodiment of the invention offers the advantage that almost all ofthe reserve power, which is provided for both the gas part and the steampart, can be made available within seconds. In particular, the reservepower attributable to the steam part can, according to an embodiment ofthe invention, be made available within seconds. There is, therefore, nolonger a delayed reaction of the steam turbine part, as mentioned in theprior art. The installations can therefore maintain the reserve power athigher unit power and the activation of the control reserve withinseconds is dynamically improved by the contribution of the steamturbine. In addition to the increased economy due to the participationin the frequency regulation, operators of combined gas and steam turbineinstallations will also, therefore, find electricity generation athigher block power more profitable.

This advantage may be achieved by a frequency maintenance power reservebeing built up for the steam part in the boiler reservoir. For thispurpose, the pressure stage is operated with a throttled control valve.Because of the throttling procedure, a back pressure builds up beforethe control valve until an equilibrium condition is reached between theinstantaneous pressure and the mass flow. In the case of underfrequency,the frequency maintenance power reserve is used for frequencymaintenance by required value correction being formed which isdetermined by the amount by which the frequency is too low. The requiredvalue correction relates to the required value of the control parameterwhich, in the relevant gas and steam turbine installation, is used forrequired/actual control. This means that a required value correction isformed, as a function of the amount by which the current frequency isless than the required frequency, which required value correctioncounteracts the frequency discrepancy.

The required value correction therefore corresponds to an increasedpassage cross section relative to the throttled condition of the controlvalve. It acts, using an applied signal decaying with time, on thepassage cross section of the relevant control valve. By this, therelevant passage cross section is increased by an amount correspondingto the required value correction in accordance with the variation withtime of the applied decaying signal, i.e. the throttling is, to thisextent, canceled. In consequence, the stored frequency-support powerreserve is activated (at least partially) for frequency maintenance inthe case of underfrequency.

The throttling and the cancellation of the throttling are thereforeeffected by way of the valve setting. In this connection, a throttlingdevice that the passage cross section of the control valve is restrictedand, due to the increase in pressure before the valve, the same massflow as flowed before the throttling continues to flow when the valve isfully open.

In this arrangement, the signal decaying with time is used to limit theduration and magnitude of the throttling cancellation mentioned. Thiscorresponds to a temporary amplification of the required valuecorrection. The signal decaying with time starts at a finite value andfalls to zero after a predetermined time. In this arrangement, thedecaying signal can, for example, be multiplied by the required valuecorrection so that the result (by means of an opening regulator, forexample) acts on the relevant control valve. In consequence, theresulting overall signal, which corresponds to the extent to which thethrottling mentioned above is canceled, advantageously falls to zeroafter the predetermined duration so that the condition without requiredvalue correction is then restored.

The signal decaying with time is dimensioned with respect to itsmagnitude, its variation and its duration in such a way that, takingaccount of the response behavior of the gas and steam turbineinstallation to an increased passage cross section, a stable operatingcondition is ensured with the required value correction. This takesaccount of the fact that the passage cross section of the control valvecan only be increased to such an extent and for so long that thepressure does not fall excessively. This will be considered in moredetail later. The participation of the steam part in the frequencymaintenance, by which means the advantages mentioned above can beachieved, is possible for the first time due to the invention. It is,therefore, likewise possible with the invention, for the first time, fora gas and steam turbine installation to participate in a more economicmanner in the frequency control, using both the gas turbine part and thesteam turbine part. By this, the load on the gas turbine part is reducedby the participation of steam turbine part in the case of underfrequencyand no longer needs to provide, by itself, the total power reservenecessary for frequency maintenance in the first seconds.

For gas and steam turbine installations, the method according to anembodiment of the invention offers the additional advantage that theextra power necessary for frequency maintenance in the case ofunderfrequency is not provided by power being supplied just at themoment of demand but has already been provided previously by a temporaryslight increase in power of the gas turbine part. The temporary slightincrease in the power proportion necessary for building up the steamreservoir reserve is not, however, lost but is actually reused, in thecase of frequency maintenance, by being released from the steamreservoir. Compensation for the temporarily smaller steam turbine powerduring the build-up of the steam reservoir reserve can be providedwithout difficulty by the gas turbine power by means of a power controlsystem which applies to the complete block so that, furthermore, the sumof the power demands on the block can always be met.

It is proposed that the control parameter relating to power should bethe upstream pressure present in the region of the control valve, whichupstream pressure is determined by measuring the steam throughput andconverting it with the aid of a modified sliding pressurecharacteristic, which is characteristic of the pressure stage andcorresponds to a throttled control valve. The valve setting is thereforedetermined from the modified sliding pressure characteristic. In thissystem, a required pressure value is calculated which represents thepressure value to be set within the pressure stage. The relationshipbetween the instantaneous steam throughput and the pressure is providedby the modified sliding pressure characteristic which is characteristicof the pressure stage. In this system, the modified sliding pressurecharacteristic is referred to a passage cross section reduced relativeto the fully opened control valve (natural sliding pressurecharacteristic).

In this way, a power reserve is available in the pressure stage at themodified sliding pressure operating point. In the case ofunderfrequency, this power reserve can, according to the invention, beused for frequency maintenance by a controlled increase in the passagecross section of the control valve.

The signal decaying with time is a decaying signal which decays with atime constant. In the case of a decay time constant=0, the decayingsignal can also be a square wave signal. A decaying signal in which thetime constant and/or the signal shape of the decaying signal is a modelof the impulse response with time of the combined gas and steam turbineinstallation is, however, preferred. In this arrangement, the timeconstant provides the rate of decrease of the decaying signal. As anapproximation, the decaying signal has sufficiently or completelydecayed after some three to six time constants. The essential parametersof the gas and steam turbine installation are taken into account in thedecaying signal; the decaying signal therefore corresponds to the realbehavior and, in the ideal case, reflects the real behavior. The D−T₁has an abrupt rise and, subsequently, a variation decreasing with thetime constant. The D−T₂ function does not, in contrast, have an abruptrise but, rather, a continuous rise which likewise has a time constant.The time constant of the rising section, however, is substantiallysmaller than that of the falling section.

The D−T_(n) function can be mathematically represented by the followingrelationship:${D - T_{n}} = {T_{D}\frac{s}{( {1 + {sT}_{1}} )( {1 + {sT}_{2}} )\ldots}}$

-   -   where    -   T_(D) is an appropriate lead time constant,    -   s is the corresponding Laplace operator and    -   T_(n) is the corresponding time constants,        which are each characteristic of the relevant installation. A        suitable model function can be derived in this way.

The use of the D−T_(n) function ensures that the required valuecorrection acts on the passage cross section of the control valve with atime behavior which is characteristic of the gas and steam turbineinstallation.

The time constants have to be selected as a function of the storagecapacity of the steam part of the installation. For most installations,the time constants are between 10 and 200 seconds.

The block power of the combined gas and steam turbine installation ispreferably controlled jointly. If such a control system is provided, itis proposed that the required value correction, in particular withimpressed signal which vanishes with time, should be additionallyprocessed in an inhibit circuit for the block power control. The inhibitcircuit inhibits correction of the block power by means of block powercontrol where this correction would counteract a power change to thepressure stage, or to the steam turbine part, on the basis of therequired value correction, in particular with an applied signal decayingwith time. The required value correction is, in consequence, fed to thestop circuit for block power control.

Thus, when the passage cross section of the control valve is increasedon the basis of the required value correction, in particular with anapplied signal decaying with time, an increase in the block power wouldbe initially registered in the block power control system, whereupon theblock power control system would counteract the increasing block power.In this case, however, the increase in the block power is desired on thebasis of the required value correction so that the action against theblock power increase—which is the correction to the block powermentioned above—has to be inhibited in this case.

This also applies in the opposite case, namely where the passage crosssection of the control valve is reduced again. A reduction in thepassage cross section has to be considered, for example, where there isan excessively high main system frequency. The block power controlsystem—corresponding to the case mentioned above—would then register areduction in the block power and would tend to counteract it. In thiscase, the increase—which is the correction to the block power mentionedabove—must again be inhibited. With the stop circuit mentioned, anembodiment of the invention functions more effectively because areaction of the block power control system against the effect of therequired value correction—in particular with an applied signal decayingwith time—is prevented.

A control action opposing the required value control for the modifiedsliding pressure, which action would likewise oppose any storageprocedures or release procedures, is prevented by the fact that adetermination of the pressure variation and of the steam throughputvariation takes place in the pressure stage and, in the case ofopposition between the parameters mentioned above, the respectivedirection, of the required value change, which acts against the tendencyof the control parameters is inhibited. A storage procedure occursduring throttling of the control valve when the power reserve is beingbuilt up. A release procedure occurs when the throttling is canceled andthe stored power reserve is—at least partially—released for frequencymaintenance. In this case, the pressure in the pressure stage is reducedbecause of the increase in the passage cross section, the steamthroughput increasing simultaneously. In this case, therefore, the twoparameters act oppositely. The increase in the pressure requiredvalue—which would, in this case, act in the opposite direction—is theninhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail using embodiment examplesillustrated in the figures. In these:

FIG. 1 shows a schematic block diagram of a gas and steam turbineinstallation,

FIG. 2 shows a schematic block diagram of a control device for carryingout the method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematic block diagram in FIG. 1 shows a gas and steam turbineinstallation 1, shown in an embodiment with two gas turbines, each witha downstream triple-pressure waste-heat boiler and one steam turbine.The gas and steam turbine installation 1 has a gas turbine part 2 and asteam part 3. higher level block control system 10 is provided whichpermits the coordinated open-loop and closed-loop control of the totalgas and steam turbine installation 1. The gas turbine part 2 includestwo gas turbine sets 25, 26. Each gas turbine set 25, 26 has acompressor 29, a turbine part 30 and a generator 28. A gas turbinecontrol system 31 is provided to control the gas turbine set 25. The gasturbine sets 25, 26 obtain their required power values 34 from the blockcontrol system 10. An actual gas turbine power value 32 and an actualgas turbine rotational speed value 33 are stored in the gas turbinecontrol system 31. The gas turbine rotational speed is used as theactual value for the frequency control of the gas turbine set. Thesecond gas turbine set 26 is correspondingly constructed.

The waste heat from each of the two gas turbine sets 25, 26 is passedvia an exhaust gas duct 27 to the respective downstream waste-heatboilers 4, 5 of the steam part 3 in the gas and steam turbineinstallation 1. In each pressure stage of the waste-heat boilers (threepressure stages are shown), steam is generated by way of the waste heatsupplied, which steam is used in a downstream steam turbine 11, 12, 13for electricity generation. Control elements (control valves or controlflaps) 6, 7, 8 for influencing the throughput of steam through the steamturbine, which in the end determines the steam turbine power, arepresent at the steam turbine inlet in each pressure stage.

In order to determine the instantaneous main system frequency, theactual value of the steam turbine rotational speed 35 is tapped off forthe steam turbine control system 15. On the basis of the actual steamturbine rotational speed value 35 measured, any discrepancy between theinstantaneous frequency from the main system required frequency isdetermined there. The control system 15 of the steam turbine actsdynamically, i.e. temporarily, against such a discrepancy by way ofcompensation. For this purpose, the control valves of the individualpressure stages are actuated as determined by control signals, whichdepend on the frequency discrepancy. In the embodiment example shown, alive steam control valve 6 for a high-pressure stage and amedium-pressure control valve 7 for a medium-pressure stage arerespectively present. In the case of overfrequency, steam is temporarilystored in the steam system by throttling the control valves and, in thecase of underfrequency, it is temporarily released. There is,correspondingly, a temporary reduction in power or increase in power inthe steam turbine.

In order to make it at all possible to react with release from storagein the case of underfrequency, it is necessary to have previously builtup a steam reservoir reserve. This is achieved by increasing the steampressures by use of the steam turbine valves in the individual pressurestages, as specified by modified sliding pressure characteristics 9.After a certain time period for throttling the control valves 6, 7, amodified sliding pressure operating point is achieved in which the steamreservoir reserve is available. The modified sliding pressurecharacteristics 9 are specified for each pressure stage as a function ofthe steam mass flow to the steam turbine. By way of a special,appropriate recognition circuit 46, 47, its value is temporarilyinhibited during the release used for frequency maintenance so that noaction is taken, by way of the sliding pressure characteristic, againstthe release procedure (this will be considered in more detail later, seeFIG. 2).

In addition to the high-pressure partial turbine 11 and themedium-pressure partial turbine 12, a low-pressure partial turbine 13 isalso present (FIG. 1), to which, in addition to the exhaust steam fromthe medium-pressure partial turbine, steam from the low-pressure stageof the waste-heat steam generator is supplied via a low-pressure controlflap 8. The use of the low-pressure control flap for frequencymaintenance is not shown in the present exemplary embodiment.

According to an embodiment of the invention, however, the low-pressurecontrol flap 8 of the low-pressure partial turbine 13 can also be usedfor frequency maintenance.

In order to determine the modified sliding pressure characteristic, thesteam mass flow 21, 20 of the associated pressure stage is recorded andsupplied to the block control system 10 for the determination of therequired pressure values for each pressure stage. In addition, theactual pressure value 18, 19 present before the respective control valve6, 7 is tapped off and is likewise supplied to the block control system10. The control system according to an embodiment of the invention cantake place by use of the parameters mentioned, which have been measuredin the steam circuit. For this purpose, the required high-pressure value16 and the required medium-pressure value 17, as determined inaccordance with the control system, are supplied by the block controlsystem 10 (where the values mentioned are tapped off and the controlparameters can be calculated) to the steam turbine control system 15 andare used there for controlling the steam turbine power.

FIG. 2 shows a schematic block diagram of a control device for carryingout the method according to an embodiment of the invention. This is acontrol device for primary control 45, which includes of one circuit ineach case for using the high-pressure stage and the medium-pressurestage of the steam turbine of a combined gas and steam turbineinstallation. The block diagram of FIG. 2 therefore relates to a controldevice for controlling a gas and steam turbine installation as shown inFIG. 1 where, again, throttling or widening is only provided for thepassage cross sections of the control valves 6 and 7 of thehigh-pressure stage and the medium-pressure stage. The circuit can alsobe extended in a corresponding, suitable manner for the use of thecontrol flap 8 of the low-pressure stage.

In the exemplary embodiment shown, the control valve passage crosssection is determined by controlling the upstream pressure, the requiredvalue being specified by means of a modified sliding pressurecharacteristic. The upstream pressure is therefore the control parameterrelating to power. For this purpose, it is also possible to make directuse of the power of the pressure stage, where this is being determined.In the present case, the power is converted into pressure.

An on/off signal 42 can be provided which switches the frequencyinfluence, according to an embodiment of the invention, on and off. Thefrequency influence is switched off, for example, if the participation,according to an embodiment of the invention, of the steam turbine is notdesired in the frequency maintenance procedure. The switches 56 wouldthen be in the off position. If the switches 56 are in the on position,the frequency maintenance according to an embodiment of the invention isactivated.

The actual high-pressure throughput value 21 is measured and, in theevaluation circuit 59, converted into a required value by way of thesliding pressure characteristic 9. The high-pressure throttling 57specifies the amount of throttling of the control valve 6. This iscalculated by an additional required pressure value proportion beingspecified which, using the sliding pressure characteristic 9, providesin the latter a correspondingly modified sliding pressure as therequired value. The required pressure value is then fed into therequired value control system 65 and is then passed on to the steamturbine control system 43 (for the high-pressure part). The requiredvalue is converted there, by means of a control function, into a controlvalue for the corresponding control valve and then acts on the controlvalve.

The required steam turbine rotational speed value 35 is continuouslytapped off and compared 53 with the required frequency value 52 (afterthe rotational speed has been converted into the correspondingfrequency). In this procedure, the frequency discrepancy is evaluated byway of a static characteristic 54 provided, which gives thecharacteristic of the required pressure value correction as a functionof the frequency discrepancy present. In this arrangement, the staticcharacteristic 54 can have a predetermined dead band; if the frequencydiscrepancy lies within the dead band, the required pressure valuecorrection is equal to zero. In the dynamic block 55, the requiredpressure value correction receives, in impressed form, a signal decayingwith time.

If the switch 56 is in the on position, the required pressure valuecorrection determined in this way is passed on via the output 38 (atwhich is present the required pressure value correction dynamicallyevaluated by the signal decaying with time) to the pressure controlsystem corresponding to the pressure stage. By this, the discrepancy canbe taken into account in this pressure control system on the basis ofthe required value correction. This results in the pressure controlsystem remaining “at rest” due to the application of the required valuecorrection and not counteracting the change in the actual value. Therequired pressure value correction with the applied signal decaying withtime—evaluated by an evaluation factor 62 which converts the requiredpressure value correction into the corresponding valve setting—is passedon simultaneously via the output 39 to the pressure control systemoutput of the high-pressure part of the steam turbine. This effects thecorresponding adjustment to the setting value of the control valve.

The dynamically evaluated required pressure value correction—evaluatedby the evaluation factor 61 which converts the required pressure valuecorrection into the corresponding valve setting—is correspondinglypresent at the medium-pressure control output 40. The controldiscrepancy is also corrected for the medium-pressure stage by thedynamically and statically evaluated required pressure value correction41 being present in the pressure control system and therefore likewiseholding the pressure control system for the medium-pressure stage “atrest”.

In addition, an inhibit circuit 48 is provided for block power control,in which is processed the required value correction by way of theapplied signal, decaying with time. This circuit 48 stops thatcorrection to the block power which counteracts the change in power ofthe pressure stage, or of the steam turbine part, on the basis of therequired value correction using the applied signal decaying with time.This is represented by the binary signals, “higher” stop 49 and “lower”stop 50. The corresponding stop signals act on the required valuecontrol system 51 of the block power control system, which transmits, inaccordance with the required block power value 36, a required powervalue for the gas turbine(s) at the control system output 37 forcontrolling the power.

Using the actual high-pressure value 18, the recognition of the opposingnature of the actual high-pressure value variation and the actual massflow value variation in the pressure stage takes place in ahigh-pressure recognition circuit 46. When the parameters mentionedabove are in opposition, the respective direction of the required valuechange acting against the tendency of the actual high-pressure value isinhibited. In detail, a quotient of these parameters is formed, which isused to determine whether the parameters are developing in opposition.The required value control system 65 is then respectively inhibited inthe manner mentioned above.

In addition, a medium-pressure throttling 58, which can likewise beswitched on and off by means of a switch 56, also takes place. Thereagain, the calculation of the pressure takes place by use of anevaluation circuit 59 using the modified sliding pressure characteristic9. For the medium-pressure part, however, the actual medium-pressurethroughput value 20 is present at the input and the result of therequired block pressure value control system 44 is present at theoutlet. A recognition circuit 47, which functions by analogy with thehigh-pressure recognition circuit 46 mentioned above, is also providedfor the medium-pressure part. In this case, the actual medium-pressurevalue 19 is present at the input.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A control method for a combined gas and steam turbine installation,having a gas turbine part and steam part with at least one pressurestage whose working steam is generated by at least one waste-heat boilerfed with waste heat from the gas turbine part and whose control valvepassage cross section is adjusted by a control system, whose requiredvalue is controlled by use of a control parameter, relating to power, ofthe pressure stage, comprising: operating the pressure stage with acontrol valve throttled in such a way that a frequency-support powerreserve is built up which, in the case of under-frequency, is used forfrequency maintenance by a required value correction being formed whichis determined by the amount by which the frequency is too low, whereinthe required value correction corresponds to an increased passage crosssection relative to the throttled condition of the control valve andacts, using an applied signal decaying with time, on the passage crosssection of the control valve; and dimensioning the signal decaying withtime in such a way that, taking account of the response behavior of thegas and steam turbine installation to the increased passage crosssection, a stable operating condition is ensured despite required valuecorrection.
 2. The method as claimed in claim 1, wherein the controlparameter relating to power is the upstream pressure present in theregion of the control valve, the upstream pressure being determined bymeasuring the steam throughput and converting it with the aid of amodified sliding pressure characteristic, which is characteristic of thepressure stage and corresponds to a throttled control valve.
 3. Themethod as claimed in claim 1, wherein the signal decaying with time isat least one of a square wave signal and a decaying signal which decayswith a time constant, at least one of the time constant and the signalshape of the decaying signal being a model for the combined gas andsteam turbine installation.
 4. The method as claimed in claim 3, whereinthe signal decaying with time corresponds to a D−T_(n) model functionfor the combined gas and steam turbine installation as follows:${D - T_{n}} = {T_{D}\frac{s}{( {1 + {sT}_{1}} )( {1 + {sT}_{2}} )\ldots}}$where T_(D) is a lead time constant, s is the corresponding Laplaceoperator and T_(n) is the corresponding time constants.
 5. The method asclaimed in claim 3, wherein at least one of the time constant and thelength of the square wave signal is between 10 and 200 seconds.
 6. Themethod as claimed in claim 1, wherein block power control of thecombined gas and steam turbine installation takes place and wherein therequired value correction, in particular with an applied signal decayingwith time, is additionally processed in an inhibit circuit of the blockpower control system which inhibits correction of the unit power whichcounteracts the power change to the pressure stage, or to the steamturbine part, on the basis of the required value correction, inparticular with an applied signal decaying with time.
 7. The method asclaimed in claim 1, wherein a determination of the pressure variationand of the mass flow variation takes place in the pressure stage and, inthe case of opposition between the parameters, the respective direction,of the required value change, which counteracts the tendency of thecontrol parameter is inhibited.
 8. The method as claimed in claim 2,wherein the signal decaying with time is at least one of a square wavesignal and a decaying signal which decays with a time constant, at leastone of the time constant and the signal shape of the decaying signalbeing a model for the combined gas and steam turbine installation. 9.The method as claimed in claim 8, wherein the signal decaying with timecorresponds to a D−T_(n) model function for the combined gas and steamturbine installation as follows:${D - T_{n}} = {T_{D}\frac{s}{( {1 + {sT}_{1}} )( {1 + {sT}_{2}} )\ldots}}$where T_(D) is a lead time constant, s is the corresponding Laplaceoperator and T_(n) is the corresponding time constants.
 10. The methodas claimed in claim 4, wherein at least one of the time constant and thelength of the square wave signal is between 10 and 200 seconds.
 11. Themethod as claimed in claim 1, wherein block power control of thecombined gas and steam turbine installation takes place and wherein therequired value correction with an applied signal decaying with time, isadditionally processed in an inhibit circuit of the block power controlsystem which inhibits correction of the unit power which counteracts thepower change to at least one of the pressure stage and to the steamturbine part, on the basis of the required value correction.
 12. Themethod as claimed in claim 1, wherein block power control of thecombined gas and steam turbine installation takes place and wherein therequired value correction with an applied signal decaying with time, isadditionally processed in an inhibit circuit of the block power controlsystem which inhibits correction of the unit power which counteracts thepower change to at least one of the pressure stage and the steam turbinepart, on the basis of an applied signal decaying with time.
 13. Themethod as claimed in claim 1, wherein block power control of thecombined gas and steam turbine installation takes place and wherein therequired value correction is additionally processed in an inhibitcircuit of the block power control system which inhibits correction ofthe unit power which counteracts the power change to at least one of thepressure stage and to the steam turbine part, on the basis of an appliedsignal decaying with time.
 14. A control method for a combined gas andsteam turbine installation, having a gas turbine part and steam partwith at least one pressure stage, whose control valve passage crosssection is controlled by use of a control parameter, relating to power,of the pressure stage, comprising: throttling the control valve, tooperate the pressure stage, such that a frequency-support power reserveis built up; using, in the case of under-frequency, thefrequency-support power reserve for frequency maintenance; determining arequired value correction based upon an amount by which the frequency istoo low, wherein the required value correction corresponds to anincreased passage cross section relative to the throttled condition ofthe control valve and acts, using an applied signal decaying with time,on the passage cross section of the control valve; and dimensioning thesignal decaying with time in such a way that, taking account of theresponse behavior of the gas and steam turbine installation to theincreased passage cross section, a stable operating condition is ensureddespite required value correction.
 15. The method as claimed in claim14, wherein the control parameter relating to power is the upstreampressure present in the region of the control valve, the upstreampressure being determined by measuring the steam throughput andconverting it with the aid of a modified sliding pressurecharacteristic, which is characteristic of the pressure stage andcorresponds to a throttled control valve.
 16. The method as claimed inclaim 14, wherein the signal decaying with time is at least one of asquare wave signal and a decaying signal which decays with a timeconstant, at least one of the time constant and the signal shape of thedecaying signal being a model for the combined gas and steam turbineinstallation.
 17. The method as claimed in claim 16, wherein the signaldecaying with time corresponds to a D−T_(n) model function for thecombined gas and steam turbine installation as follows:${D - T_{n}} = {T_{D}\frac{s}{( {1 + {sT}_{1}} )( {1 + {sT}_{2}} )\ldots}}$where T_(D) is a lead time constant, s is the corresponding Laplaceoperator and T_(D) is the corresponding time constants.
 18. The methodas claimed in claim 16, wherein at least one of the time constant andthe length of the square wave signal is between 10 and 200 seconds. 19.The method as claimed in claim 14, wherein block power control of thecombined gas and steam turbine installation takes place and wherein therequired value correction, in particular with an applied signal decayingwith time, is additionally processed in an inhibit circuit of the blockpower control system which inhibits correction of the unit power whichcounteracts the power change to the pressure stage, or to the streamturbine part, on the basis of the required value correction, inparticular with a signal decaying with time.
 20. The method as claimedin claim 14, wherein a determination of the pressure variation and ofthe mass flow variation takes place in the pressure stage and, in thecase of opposition between the parameters, the respective direction, ofthe required value change, which counteracts the tendency of the controlparameter is inhibited.